Just a moment of anger n sadness, well..... i sense that she is
behaving werid during those few days and one of those day we
quarrel and i decided to break up.
The question is, do you have any evidence of what she is doing
out there?
It could be totally due to your own suspicions and unwillingness
to trust her that result in you breaking up with her.
But the main thing you must understand is that you are the one
who decided to dump her.
Quite a number of girls are not as strong as they will wish for
themselves to be - they need someone to be a pillar of support for
them. Once you have thrown her away, she will realise that there is
no one there to pull her up; and thus she went and look for another
boyfriend. And this boyfriend could just be someone who has always
wanted to chase after her.
To be fair to you, indeed, there is the chance that she was
cheating behind your back, but you shouldn't have ask to break up
so easily. Now, you have become the "bad guy", no matter whether
you regret your actions or not.